Basic Concepts in Refrigeration

Basic Concepts in Refrigration

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Introduction

• Refrigeration is concerned with the production of cool confinement absorbing heat from the space where cooling is required
• The branch of science which deals with the process of reducing and maintaining the temperature of a space or material below the temperature of the surrounding
• The heat is then rejected to some natural sink such as:
• The atmospheric air
• Surface water
• Any external body lower in temperature compared to the space

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Continued………

• A refrigeration system is a combination of components, equipment and piping connected in a sequence to produce the refrigeration effect
• Refrigeration Cycle: when a refrigerant undergoes a series of processes like evaporation, compression, condensation and expansion, it is said to have undergone a refrigeration cycle

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Continued……

• Refrigeration cycle are classified mainly in to the following :
• Vapor compression refrigeration cycle
• Vapor absorption refrigeration cycle
• Air refrigeration cycle and
• Steam- jet refrigeration cycle

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Principles of Refrigeration

REFRIGERATION SYSTEM COMPONENTS

The basic components of a refrigeration system are:

- Evaporator

- Compressor

- Condenser

- Expansion Valve

- Refrigerant; to conduct the heat from the product

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Evaporator

• The purpose of the evaporator is to remove unwanted heat from the product
• Refrigerant contained within the evaporator is boiling at a low-pressure. The level of this pressure is determined by two factors:

- The rate at which the heat is absorbed from the product to the liquid refrigerant in the evaporator

- The rate at which the low-pressure vapor is removed from the evaporator by the compressor

• When leaving the evaporator coil the liquid refrigerant is in vapor form.

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Compressor

• The purpose of the compressor is to draw the low-temperature, low-pressure vapor from the evaporator via the suction line.
• When vapor is compressed it rises in temperature.
• The compressor transforms the vapor from a low-temperature vapor to a high-temperature vapor, in turn increasing the pressure.

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Condenser

• The purpose of the condenser is to extract heat from the refrigerant to the outside air.
• Fans mounted above the condenser unit are used to draw air through the condenser coils.
• The temperature of the high-pressure vapor determines the temperature at which the condensation begins.
• As heat has to flow from the condenser to the air, the condensation temperature must be higher than that of the air.
• The high-pressure vapor within the condenser is then cooled to the point where it becomes a liquid refrigerant

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Expansion Valve

• The expansion valve is located at the end of the liquid line, before the evaporator. The high-pressure liquid reaches the expansion valve, which come from the condenser.
• The valve then reduces the pressure of the refrigerant as it passes through the orifice, which is located inside the valve.
• On reducing the pressure, the temperature of the refrigerant also decreases to a level below the surrounding air.
• This low-pressure, low-temperature liquid is then pumped in to the evaporator.

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Need for Refrigeration

• The growth of microorganisms is temperature-dependent, that growth declines as temperature falls, and that growth becomes very slow at temperatures below +10 °C
• Use of refrigeration to conserve foodstuffs

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Application of Refrigeration

• Foodstuff production, conservation and preservation
• Chemical processing industry
• Industrial and comfort air conditioning plants
• Drying plants, etc

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Vapor absorption cycle

• Some liquids like water have great affinity

for absorbing large quantities of certain

vapors (NH₃) and reduce the total volume greatly.

• The absorption refrigeration system differs fundamentally from vapor compression system only in the method of compressing the refrigerant.

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• An absorber, generator and pump in the absorption refrigerating system replace the compressor of a vapor compression system.
• Figure shows the schematic diagram of a vapor absorption system.
• Ammonia vapor is produced in the generator at high pressure from the strong solution of NH₃ by an external heating source.

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• The water vapor carried with ammonia is removed in the rectifier and only the dehydrated ammonia gas enters into the condenser.
• High pressure NH₃ vapor is condensed in the condenser.
• The cooled NH₃ solution is passed through a

throttle valve and the pressure and temperature of the refrigerant are reduced below the temperature to be maintained in the evaporator.

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• The low temperature refrigerant enters the evaporator and absorbs the required heat from the evaporator and leaves the

evaporator as saturated vapor.

• Slightly superheated, low pressure NH₃

vapor is absorbed by the weak solution of NH₃

which is sprayed in the absorber as shown in

Fig.

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• Weak NH₃ solution (aqua–ammonia) entering the absorber becomes strong solution after absorbing NH₃ vapor and then it is pumped to the generator through the heat exchanger.
• The pump increases the pressure of the strong solution to generator pressure.
• The strong NH₃ solution coming from the absorber absorbs heat from high temperature weak NH₃ solution in the heat exchanger

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• The solution in the generator becomes weak as NH₃ vapor comes out of it. The weak high temperature ammonia solution from the generator is passed to the heat exchanger through the throttle valve.
• The pressure of the liquid is reduced to the absorber pressure by the throttle valve.

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Comparison between Vapor Compression and Absorption system

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Comparison between Vapor Compression and Absorption system

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Methods of Refrigeration

• There are different methods of refrigeration. Among them:
• Solution (dissolution of salts in water) and
• Change of Phase

Are mostly used in the refrigeration processes

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Solution (dissolution of Salts in water)

• When certain salts such as NaCl or CaCl₂ are dissolved in water, they absorb heat

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• CaCl₂ lowers the temperature of water up to -50^oC while NaCl up to -20^oC

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• The salts used for refrigeration has to be regained by evaporating the solution

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• The refrigeration produced is quite small compared to the large amount of energy required in salt regaining process

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Change of Phase

• If a substance such as ice is available, it is possible to get refrigeration due to the change of phase
• Solid change to liquid, the cooling produced is:

Q_s1=m₁h_sf

Where m1 is the rate of fusion of ice &

h _{s f} is the enthalpy of fusion (340kJ/kg)

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Continued………

• Refrigeration can be produced by change of phase from solid to vapor-Sublimation. This occurs when the pressure of the system is lower than the triple point pressure

Q_c2=m₂h_sv

Where h _{s v} is the enthalpy of sublimation

• Solid carbon dioxide (dry ice) at one atmospheric pressure produces about 570kJ/Kg of refrigeration maintaining it self at a temperature of about -75 °C

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Continued………

• Refrigeration can be created due to the phase transformation from liquid to vapor

Q_c3=m₃ (h_g-h _f)

=m₃h_fg

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Introduction

• Refrigerant : is the primary working fluid used for absorbing and transmitting heat in a refrigeration system
• Refrigerants absorb heat at low temperature and low pressure and release heat at a higher temperature and pressure
• Most refrigerants undergo phase changes during heat absorption (evaporation) and heat releasing (condensation)

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Selection of refrigerant

• Operating temperatures.
• System design, size, initial and operating costs, safety, reliability, and serviceability etc.
• Due to several environmental issues such as ozone layer depletion, global warming and their relation to the various refrigerants used, the selection of suitable refrigerant has become one of the most important issues in recent times.

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• Replacement of an existing refrigerant by a completely new refrigerant, for whatever reason, is an expensive proposition as it may call for several changes in the design and manufacturing of refrigeration systems.
• In principle, any fluid can be used as a refrigerant.
• Air used in an air cycle refrigeration system can also be considered as a refrigerant.

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Classification

• Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants.
• Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapor compression and vapor absorption refrigeration systems.
• When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator.

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• Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other.
• Secondary refrigerants are also known under the name brines or antifreezes.
• Antifreezes or brines are used when refrigeration is required at sub-zero temperatures.
• Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other.
• The commonly used secondary refrigerants are the solutions of water, ethylene glycol, propylene glycol and calcium chloride. These solutions are known under the general name of brines.

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• Cooling Medium: is a working fluid cooled by the refrigerant to transport the cooling effect between a central plant and remot cooling units and terminals
• Chilled water, brine, and glycol are used as cooling media in many refrigeration systems.
• The cooling medium is often called a secondary refrigerant, because it reduces the extensive circulation of the primary refrigerant

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Classification of Refrigerants

• The National Refrigeration Safety Code, USA (NRSC) catalogues all the refrigerants into three groups. Some of these are:
• Group one – ( Safest of the refrigerants)

R-113, R-11, R-21, R-114, R-12, R-30, R-22, R-744, R-502, R-13, R-14, R-500, R-134a

• Group two- (Toxic and somewhat Inflammable Refrigerants)

R-1130, R-611, R-160, R-764, R-40, R-717

• Group Three- (inflammable Refrigerants)

R-600, R-290, R-170, R-1150, R-50

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Refrigeration effect and unit of refrigeration

• The rate at which heat is absorbed from space to be cooled is termed as the refrigeration effect
• The refrigeration effect can be compared with the production of ice
• In the refrigeration industry, the unit used is ton
• A ton of refrigeration is equivalent to the rate of heat transfer needed to produce 1 ton of ice from water at 0 ^oC in 24 hours
• A machine capable of producing a net refrigeration effect of 210 kJ/min or 3.5 kW is called a 1 ton machine

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Economic properties:

• The refrigerant used should preferably be inexpensive and easily available.

Designation of refrigerants:

• Since a large number of refrigerants have been developed over the years for a wide variety of applications, a numbering system has been adopted to designate various refrigerants.
• From the number one can get some useful information about the type of refrigerant, its chemical composition, molecular weight etc.
• All the refrigerants are designated by R followed by a unique number.

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1. Fully saturated, halogenated compounds:
1. These refrigerants are derivatives of alkanes (C_nH_2n+2) such as methane (CH₄), ethane (C₂H₆).
2. These refrigerants are designated by R XYZ,

where:

• X+1 indicates the number of Carbon (C) atoms
• Y-1 indicates number of Hydrogen (H) atoms, and
• Z indicates number of Fluorine (F) atoms
• The balance indicates the number of Chlorine atoms.
• Only 2 digits indicates that the value of X is zero.

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R 22

• X = 0 ⇒ No. of Carbon atoms = 0+1 = 1 ⇒ derivative of methane (CH₄)
• Y = 2 ⇒ No. of Hydrogen atoms = 2-1 = 1
• Z = 2 ⇒ No. of Fluorine atoms = 2
• The balance = 4 – no. of (H+F) atoms = 4-1-2 = 1 ⇒ No. of Chlorine atoms = 1

∴The chemical formula of R 22 = CHClF₂

• Similarly it can be shown that the chemical formula of:
• R12 = CCl₂F₂
• R134a = C₂H₂F₄ (derivative of ethane)
• (letter a stands for isomer, e.g. molecules having same chemical composition but different atomic arrangement, e.g. R134 and R134a)

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2. Inorganic refrigerants: These are designated by number 7 followed by the molecular weight of the refrigerant (rounded-off).
1. Ex.: Ammonia: Molecular weight is 17, ∴ the designation is R 717
2. Carbon dioxide: Molecular weight is 44, ∴ the designation is R 744
3. Water: Molecular weight is 18, ∴ the designation is R 718

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3. Mixtures: Azeotropic mixtures are designated by 500 series, where as zeotropic refrigerants (e.g. non-azeotropic mixtures) are designated by 400 series.

Azeotropic mixtures:

• R 500: Mixture of R 12 (73.8 %) and R 152a (26.2%)
• R 502: Mixture of R 22 (48.8 %) and R 115 (51.2%)
• R503: Mixture of R 23 (40.1 %) and R 13 (59.9%)
• R507A: Mixture of R 125 (50%) and R 143a (50%)

Zeotropic mixtures:

• R404A : Mixture of R 125 (44%), R 143a (52%) and R 134a (4%)
• R407A : Mixture of R 32 (20%), R 125 (40%) and R 134a (40%)
• R407B : Mixture of R 32 (10%), R 125 (70%) and R 134a (20%)
• R410A : Mixture of R 32 (50%) and R 125 (50%)

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Hydrocarbons:

• Propane (C₃H₈) : R 290
• n-butane (C₄H₁₀) : R 600
• iso-butane (C₄H₁₀) : R 600a
• Unsaturated Hydrocarbons: R1150 (C₂H₄)

R1270 (C₃H₆)

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Requirements of Refrigerant

• There are certain desirable characteristics which a fluid used as a refrigerant should posses:
• Non-poisonous
• Non-explosive
• Non-corrosive
• Non-inflammable
• Leaks should be easily detected
• Leaks should be easy to locate
• Should operate under low pressure
• Stable gas

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Continued……..

• Parts moving in the fluid should be easy to lubricate
• Non- toxic
• Well balanced enthalpy of evaporation per unit mass
• A minimum difference between the vaporizing and condensing pressure is desirable
• The standard comparison of refrigerants as used in refrigeration industry is based on an evaporator temperature of -15^oC and condensing temperature of 30^oC

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Identification Refrigerant by Number

Refrigerant No Name and chemical formula

R-11 Trichloromonofluoromethane (CCl₃F)

R-12 Dichlorodifluoromethane (CCl₂F₂)

R-22 Monochlorodifluoromethane (CHClF₂)

R-500 Mixture of 73.8% R-12 and 26.2% R-152a

R-502 Mixture of 48.8% R-22 and 51.2% R-115

R-717 Ammonia (NH₃)

R-134a Tetrafluoroethane (CH₂FCF₃)

R-152a Difluorethane

R-115 Chloropentafluoroethane

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